1. Field of the Invention
The present invention relates to thrust bearings. More particularly, the present invention provides a thrust bearing of the type comprising a pad carrier coupled to an annular series of discrete thrust pads having respective thrust bearing surfaces against which rotatably bears a co-operating annular bearing or load surface of a rotating member. More particularly further, the present invention also provides a method for equalizing a load on the thrust pads.
2. Description of the Prior Art
There is seen in FIG. 1 a prior art end-thrust bearing, generally illustrated as 10. The end-thrust bearing 10 includes a collar 12 supporting a plurality of stationary segments 14. The segments 14 in the actual bearing are pivoted in order to allow them to assume slightly different angles by which to create different degrees of convergence of respective oil films 16. The collar 12 is attached to a rotating member (not shown). Oil is introduced at the inner edges of the segments 14 so that the centrifugal action of the rotating member 19 causes the oil to flow radially outward, at the same time the oil adhering to the surface of the collar moves circumferentially and builds up pressure in the film. The wedge formation of the oil film 16 insures a complete separation of the metal parts. The prior art bearing 10 in FIG. 1 may be made in types suitable for vertical or horizontal shafts, for carrying thrusts in either or both longitudinal directions, and for the dissipation of large quantities of heat by fan or water cooling. Segments 14 are spaced around the periphery of the collar 12, and each segment 14 is supported so that it can tip to form the proper wedge shape for the oil film 16.
Thrust bearings, such as the one in FIG. 1, are presently imposed with high load and performance expectations because of increasing performance, price, and reliability demands for advanced machinery, such as modem high-speed compressors, gas turbines, steam turbines, gearboxes, and the like. Of the variety of fluid-film bearing designs employed to support thrust loads, tilting-pad designs offer the highest load capacity. Due to manufacturing tolerances, misalignment, shaft flexibility and related dynamic forces, equalization of the load carried by the individual pads is required to reliably obtain the maximum load capacity. The equalization of mechanically-pivoted thrust pads typically involves tight tolerancing, mechanical linkages, or flexible members, which introduce several disadvantages into the design. Tight tolerancing may add excessive cost to the pads and supporting hardware and is only an approximate means of equalization. Mechanical linkages require considerable axial space and are known to lock-up, ceasing to provide equalization under high loads or certain forms of dynamic loads. Furthermore, the contact points of the linkages are subject to wear. Flexible members are subject to wear at the contact points and have the potential for fatigue failure.
An improved tilting-pad thrust bearing was developed which uses hydrodynamic pressure generated in a lubricant by the rotation of the journal over the surface of the pad as a source for hydrostatic pressure to support each pad independently. Such a hydrostatically-supported tilting-pad thrust bearing is disclosed in U.S. Pat. No. 3,982,796 to Hill, which teaches applying lubricant to the leading edges of each pad. The thrust bearing disclosed in U.S. Pat. No. 3,982,796 to Hill provides for load equalization through the hydrostatic suspension of individual pads and allows for misalignment accommodation by suspension of the carrier ring using a fraction of the hydrostatic lubricant. While each individual pad is taught to possess respective load equalization ability, there is no teaching of load equalization ability between two or more pads in conjunction. Thus, there remains uneven load equalization on all pads.
With respect to load equalization, mechanical designs of conventional thrust bearings are subject to contact fretting, deformation, and wear, or to fatigue damage. Mechanical designs are further limited in their ability to respond to dynamic changes in relative pad loading. The current use of hydrostatic suspension for the load equalization of tilting-pad thrust bearings is limited to individual pads, which again does not accommodate dynamic changes in relative pad loading. Therefore, what is needed and what has been invented is an improved thrust bearing which does not possess the deficiencies of conventional thrust bearings. More particularly, what is needed and what has been invented is a thrust bearing and method for equalizing load on thrust pads of a thrust bearing.
Embodiments of the present invention provide a thrust bearing comprising a pad carrier having a pad-carrying surface, a sealing-plate surface, and a carrier structure defining a lubricant manifold, a pressure equalization manifold, at least two pressure equalization passages communicating with and extending from the pressure equalization manifold and terminating in the pad-carrying surface, and at least two lubricant passages communicating with and extending from the lubricant manifold and terminating in the pad-carrying surface. The thrust bearing also comprises at least two thrust pads supported by the pad-carrying surface. Each thrust pad has a pad structure defining a bearing surface and a rear pad surface, at least a portion of which communicates with one of the pressure equalization passages. Each thrust pad also includes a pad passage that extends from the rear pad surface and communicates with the bearing surface, and a lubricant inlet bore that communicates with one of the lubricant passages of the pad carrier.
In another embodiment of the present invention, a thrust bearing assembly is provided having a receptacle containing a lubricant and a thrust bearing immersed in the lubricant. The thrust bearing comprises a pad carrier having a pad-carrying surface, a sealing-plate surface, and a carrier structure defining a pressure equalization manifold, at least two pressure equalization passages communicating with and extending from the pressure equalization manifold and terminating in the pad-carrying surface. At least two thrust pads are supported by the pad-carrying surface. Each thrust pad has a pad structure defining a bearing surface, a rear pad surface with at least a portion of which communicates with one of the pressure equalization passages, and a pad passage extending from the rear pad surface and communicating with the bearing surface.
The rear pad surface of each thrust pad may define a rear pad recess. In one embodiment of the invention, the pad-carrying surface may define a plurality of carrier recesses. The rear pad recess is generally opposed to a pressure equalization passage when each of the thrust pads is supported by the pad-carrying surface. The carrier structure may additionally define a sealing plate recess. A sealing plate is preferably disposed in the sealing plate recess such as to isolate and seal the pressure equalization manifold. A pressure sensor member may be disposed through the sealing plate to indicate a pressure within the pressure equalization manifold.
Embodiments of the present invention further provide a method for equalizing loads between two or more thrust pads of a thrust bearing. The method broadly comprises providing a thrust bearing including at least two thrust pads with each thrust pad having a rear pad surface and supported by a pad carrying surface of a pad carrier; disposing a lubricant on the thrust pads; rotating a bearing element against the thrust pads such that each thrust pad has a different load; and transferring a portion of the lubricant from one volumetric space between one rear pad surface and the pad carrying surface to a volumetric space between another rear pad surface and the pad carrying surface in order to equalize local pressures, and thereby loads between the thrust pads. The method preferably additionally comprises transferring (preferably prior to transferring a portion of the lubricant between volumetric spaces) a portion of the lubricant from a load surface of one thrust pad to a rear pad surface thereof, whereby, due to a different load on each thrust pad, a different pressure develops at the rear pad surface of each rear pad surface. The pad carrier includes a carrier structure defining an equalization manifold, and the transferring of a portion of the lubricant from one volumetric space to another volumetric space comprises passing the portion of the lubricant through an equalization manifold. The method may additionally comprise measuring a pressure of the lubricant within the equalization manifold.
These provisions together with the various ancillary provisions and features which will become apparent to those skilled in the art as the following description proceeds, are attained by the thrust bearing and method of the present invention, preferred embodiments thereof being shown with reference to the accompanying drawings, by way of example only, wherein: